Add calculations to apply hover over high importance buttons

Fixes fyne-io#1785

widget/button.go

@@ -312,10 +312,20 @@ func (r *buttonRenderer) buttonColor() color.Color {
 	switch {
 	case r.button.Disabled():
 		return theme.DisabledButtonColor()
-	case r.button.Importance == HighImportance:
-		return theme.PrimaryColor()
 	case r.button.hovered:
+		if r.button.Importance == HighImportance {
+			srcR, srcG, srcB, srcA := theme.PrimaryColor().RGBA()
+			dstR, dstG, dstB, dstA := theme.HoverColor().RGBA()
+			alpha := float32(dstA) / 0xFFFF
+			outR := alpha*float32(dstR) + (1-alpha)*float32(srcR)
+			outG := alpha*float32(dstG) + (1-alpha)*float32(srcG)
+			outB := alpha*float32(dstB) + (1-alpha)*float32(srcB)
+			outA := alpha*float32(dstA) + (1-alpha)*float32(srcA)
+			return color.RGBA{R: uint8(uint32(outR) >> 8), G: uint8(uint32(outG) >> 8), B: uint8(uint32(outB) >> 8), A: uint8(uint32(outA) >> 8)}
+		}
 		return theme.HoverColor()
+	case r.button.Importance == HighImportance:
+		return theme.PrimaryColor()
 	default:
 		return theme.ButtonColor()
 	}

widget/button_test.go

@@ -121,6 +121,31 @@ func TestButton_Disabled(t *testing.T) {
 	assert.False(t, button.Disabled())
 }
 
+func TestButton_Hover(t *testing.T) {
+	app := test.NewApp()
+	defer test.NewApp()
+	app.Settings().SetTheme(theme.LightTheme())
+
+	b := widget.NewButtonWithIcon("Test", theme.HomeIcon(), func() {})
+	w := test.NewWindow(b)
+	defer w.Close()
+
+	test.MoveMouse(w.Canvas(), fyne.NewPos(5, 5))
+	test.AssertImageMatches(t, "button/hovered.png", w.Canvas().Capture())
+
+	b.Importance = widget.HighImportance
+	b.Refresh()
+	test.AssertImageMatches(t, "button/high_importance_hovered.png", w.Canvas().Capture())
+
+	test.MoveMouse(w.Canvas(), fyne.NewPos(0, 0))
+	b.Refresh()
+	test.AssertImageMatches(t, "button/high_importance.png", w.Canvas().Capture())
+
+	b.Importance = widget.MediumImportance
+	b.Refresh()
+	test.AssertImageMatches(t, "button/initial.png", w.Canvas().Capture())
+}
+
 func TestButton_Layout(t *testing.T) {
 	test.NewApp()
 	defer test.NewApp()